1. Field of the Invention
This invention relates to optical arrangements used in imaging systems. More specifically, this invention relates to optical arrangements operative to modify the fields of view of sensors included within such imaging systems.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Infrared imaging systems are used in a variety of military and commercial applications to provide either an operator or a guidance system with a view of a scene. Such imaging systems typically include a sensor arrangement capable of collecting radiant energy originating from within a field of view of the scene. The sensor arrangement will typically include several optical elements configured to focus incident scene energy upon a detector (such as a focal plane array). The optical elements are selected to provide the sensor with a desired field of view. A wide field of view is advantageous in that it allows a viewer to observe objects therein within the context of a larger scene. However, increases in the field of view result in corresponding decreases in resolution. In certain sensors these competing concerns are addressed by a compromise design having an intermediate field of view.
Various methods have been utilized in an attempt to avoid the necessity of striking a balance between image resolution and field of view. For example, in a particular optical approach the imaging system is designed to incorporate a pair of lens assemblies. One of the assemblies encompasses a wide field of view, while the other covers a relatively narrow field of view. The assemblies are then mechanically moved in and out of the optical train of the sensor to alternately provide a wide field of view or improved resolution. Unfortunately, implementation of this approach typically requires a relatively complex servo system to alternately interpose the lens assemblies in alignment with the sensors. In addition, the employment of more than a single auxiliary lens assembly raises the cost of the imaging system.
As is well known, the focusing power of a lens element is a function of the thickness, index of refraction, and radius of curvature thereof. Since the values of all of these parameters generally vary as a function of temperature, the focusing power of lens assemblies incorporated within imaging systems are subject to temperature-induced variation. In one scheme employed in an attempt to counteract this variation, a mechanical device attached to the lens assembly operates to move the constituent lens elements in response to temperature fluctuations. In this way an effort is made to minimize the impact of temperature change on the performance of the lens assembly. Unfortunately, this movement technique typically relies on a complex and expensive mechanical focus compensation device.
Accordingly, a need exists in the art for a lens assembly capable of providing multiple fields of view to an electromagnetic sensor arrangement.